Lithium-manganese oxide (LiMn2O4) has been extensively studied together with lithium-nickel oxide (LiNiO2) as an alternative to the lithium-cobalt oxide (LiCoO2) which is currently used as a cathode active material of a lithium secondary cell, for the reason that the lithium-cobalt oxide is expensive and suffers from environmental problems. Also, the use of the lithium-cobalt oxide and lithium-nickel oxide is hampered by the danger of explosion due to the oxygen generated when overcharged, but a lithium-manganese oxide does not generate oxygen even when overcharged and has a cost-merit due to the use of less expensive Mn.
However, LiMn2O4 has a lower theoretical charge capacity (148 mAh/g) than the lithium-cobalt oxide (274 mAh/g), and as the charge/discharge cycle is repeated, its discharge capacity rapidly decreases. Such poor cycle property results from the transition from the original cubic phase to a tetragonal phase due to the generation of Li2Mn2O4 on the cathode as lithium ions are intercalated during charge/discharge cycles [W. Liu el al., J. Electronchem. Soc., 1996, Vol. 143, No. 11, pp. 3590-3596; R. J. Gummow et al., Solid State Ionics, 1994, Vol. 69, pp. 59-67]. As lithium ions are intercalated, the valence of Mn becomes smaller than 3.5 or less and a strong Jahn-Teller distortion occurs, transforming the cubic crystalline phase to a tetragonal phase with which intercalation/deintercalation of lithium ions becomes more difficult [G. Pistoia et al., Solid State Ionics, 1995, Vol. 78, pp. 15-122]. In other words, the arrangement of Mn(III) (t32g·e1g, high spin) in a spinel structure changes and the octahedron becomes severely elongated and the c/a ratio increases by 16% per unit cell, causing destabilization of the cathode structure and the poor charge/discharge cycle characteristic. Furthermore, as the valence of Mn(3d4) in LiMn2O4 reverts back to 3.5 during the lithium deintercalation, the discharge becomes more difficult, and the charge/discharge capacity decreases rapidly.